Continuous-time delta-sigma ADC with programmable input range

ABSTRACT

A scaled input current is produced that substantially matches the full scale input of a CTΔτADC that substantially cancels an offset bias current component of the input current. A variable bias resistance value is coupled between the integrator input and one of a supply voltage and a circuit common. The method further includes integrating the input current to produce an integrated signal representing a time averaged value of the input current to substantially remove noise from a frequency band of interest. The integrated signal is produced to a quantizer to produce a feedback current that substantially cancels a quantization noise component in the digital representation of the scaled analog signal by coupling the digital representation of the scaled analog signal to a programmable digital switch wherein the programmable digital switch either sinks current from or sources current to the integrator input.

BACKGROUND

1. Technical Field

The present invention relates to communication systems and, moreparticularly, to analog-to-digital and digital-to-analog converters usedwithin transceivers.

2. Related Art

Communication systems are known to support wireless and wire linedcommunications between wireless and/or wire lined communication devices.Such communication systems range from national and/or internationalcellular telephone systems to the Internet to point-to-point in-homewireless networks. Each type of communication system is constructed, andhence operates, in accordance with one or more communication standard.For instance, wireless communication systems may operate in accordancewith one or more standards, including, but not limited to, IEEE 802.11,Bluetooth, advanced mobile phone services (AMPS), digital AMPS, globalsystem for mobile communications (GSM), code division multiple access(CDMA), local multi-point distribution systems (LMDS),multi-channel-multi-point distribution service (MMDS), and/or variationsthereof.

Depending on the type of wireless communication system, a wirelesscommunication device, such as a cellular telephone, two-way radio,personal digital assistant (PDA), personal computer (PC), laptopcomputer, home entertainment equipment, etc., communicates directly orindirectly with other wireless communication devices. For directcommunications (also known as point-to-point communications), theparticipating wireless communication devices tune their receivers andtransmitters to the same channel or channels (e.g., one of a pluralityof radio frequency (RF) carriers of the wireless communication system)and communicate over that channel(s). For indirect wirelesscommunications, each wireless communication device communicates directlywith an associated base station (e.g., for cellular services) and/or anassociated access point (e.g., for an in-home or in-building wirelessnetwork) via an assigned channel. To complete a communication connectionbetween the wireless communication devices, the associated base stationsand/or associated access points communicate with each other directly,via a system controller, via the public switched telephone network(PSTN), via the Internet, and/or via some other wide area network.

Each wireless communication device includes a built-in radio transceiver(i.e., receiver and transmitter) or is coupled to an associated radiotransceiver (e.g., a station for in-home and/or in-building wirelesscommunication networks, RF modem, etc.) that performs analog signalprocessing tasks as a part of converting data to a radio frequency (RF)signal for transmission and a received RF signal to data.

As is known, the transmitter includes a data modulation stage, one ormore intermediate frequency stages, and a power amplifier. The datamodulation stage converts raw data into baseband signals in accordancewith the particular wireless communication standard. The one or moreintermediate frequency stages mix the baseband signals with one or morelocal oscillations to produce RF signals. The power amplifier amplifiesthe RF signals prior to transmission via an antenna.

As is also known, the receiver is coupled to the antenna and includes alow noise amplifier, one or more intermediate frequency stages, afiltering stage, and a data recovery stage. The low noise amplifierreceives an inbound RF signal via the antenna and amplifies it. The oneor more intermediate frequency stages mix the amplified RF signal withone or more local oscillations to convert the amplified RF signal into abaseband signal or an intermediate frequency (IF) signal. As usedherein, the term “low IF” refers to both baseband and intermediatefrequency signals.

A filtering stage filters the low IF signals to attenuate unwanted outof band signals to produce a filtered signal. The data recovery stagerecovers raw data from the filtered signal in accordance with theparticular wireless communication standard. Alternate designs beingpursued at this time further include direct conversion radios thatproduce a direct frequency conversion often in a plurality of mixingsteps or stages.

As an additional aspect, these designs are being pursued as a part of adrive to continually reduce circuit size and power consumption. Alongthese lines, such designs are being pursued with CMOS technology therebypresenting problems not addressed by prior art designs. For example, onecommon design goal is to provide an entire system on a single chip. Thedrive towards systems-on-chip solutions for wireless applicationscontinues to replace traditional analog signal processing tasks withdigital processing to exploit the continued shrinkage of digital CMOStechnology.

One approach to current designs by the applicant herein is to reduceanalog signal processing performance requirements and to compensate forthe relaxed performance requirements in the digital domain to providerequired system performance. This approach is beneficial in that, inaddition to the reduced silicon area requirements, the digitalprocessing is insensitive to process and temperature variations.Applications for which this trend is observed include RF receivers wherethe received signal is digitized as early as possible in the receiverchain using a high dynamic range analog-to-digital converter (ADC), andin a variety of calibration circuits of the radio where signal levelsmust be measured accurately over a wide range of values. This trend thusincreases the demand for embedded low-power, low-voltage ADCs providinghigh dynamic range in the interface between the analog and digitalprocessing.

A class of ADCs capable of providing high dynamic range and particularlysuitable for low-power and low-voltage implementation is known ascontinuous-time delta sigma analog-to-digital converters (CTΣΔADCs).These ADCs can be designed to operate with supply voltages in the rangeof 1.2V–1.5V and current consumption as low as a few hundred μAs.

FIG. 1 shows an example top-level block diagram of the simplest CTΣΔADC,namely, the first-order low pass CTΣΔADC. The applicant specificallynotes that the discussion of FIGS. 1–3 herein the Background section isintended solely as a discussion of related technology or art and in noway constitutes an admission that the circuit shown in FIGS. 1–3 and thecorresponding discussion below is prior art.

The input signal to the CTΣΔADC of FIG. 1 is a voltage source labeleds(t). An operational amplifier with negative capacitive feedbackconstitutes an integrator formed by the operational amplifier andcapacitor in a feedback loop, which integrates the input current labeledi_(s)(t) flowing from an input signal s(t) to produce an analogintegrator output voltage. A coarse (in this example 2-bit) quantizerconverts the analog integrator output voltage signal to a digital formatshown as y(t). The quantizer, by providing a 2-bit output, defines whichof four voltage levels most closely match the analog integrator outputvoltage. More specifically, the quantizer produces a 2-bit output havingvalues of 00, 01, 10 and 11.

The quantizer typically includes an array of comparators, essentially1-bit ADCs, whose output is either “high” or “low” depending upon themagnitude of the integrator voltage relative to a reference signalgenerated by a reference generator. A digital-to-analog converter (DAC)provides a feedback current responsive to a logic value (“1” or “0”) ofADC output to the integrator. FIG. 2 shows one implementation of the2-bit quantizer that produces the 2-bit feedback to the DAC. Thequantizer sums the output values of the array of comparators to producethe 2-bit output discussed above.

FIG. 3 shows an alternative model of the first-order CTΣΔADC of FIG. 1,wherein the quantizer has been replaced with an additive noise sourceq(t). The model of FIG. 3 is one that represents the CTΣΔADC of FIG. 1.Because the operation of the quantizer is deterministic, a signal q(t)may be defined such that the CTΣΔADC of FIG. 3 behaves similarly to theCTΣΔADC of FIG. 1. The digital ADC output, here denoted y(t), can thenbe written as a sum of two terms, namely, a term related to the inputsignal, y_(s)(t), and a term related to the quantization noise,y_(q)(t), i.e.,y(t)=y _(s)(t)+y _(q)(t).  (1)

By employing feedback around the integrator and quantizer combination,it is possible to suppress the quantization noise component y_(q)(t) ina limited frequency range around DC. Specifically, it can be shown thaty_(q)(t) results from q(t) being filtered by a first-order high-passfilter, commonly referred to as the noise transfer function, NTF(s),i.e., in terms of Laplace transforms,Y _(q)(s)=NTF(s)×Q(s).  (2)

Similarly, for a low-frequency input signal s(t), it can be shown thatthe signal component y_(s)(t) equals the input signal, i.e., in terms ofLaplace transforms,Y _(s)(s)=S(s).  (3)

The above properties explain the terminology “low pass” CTΣΔADC; if s(t)is a low frequency input signal, the ADC output y(t) closely resembless(t) when considering only the low frequency region of y(t), i.e., theADC “passes” signals of low frequency from analog to digital formatwithout alteration. Furthermore, the low pass CTΣΔADC of FIG. 1 is offirst-order since the single integrator gives rise to a first order highpass filter. More integrators can, in principle, be added to yieldhigher order filtering of the quantization noise as is described furtherbelow. Generally, an N^(th) order CTΣΔADC contains N integrators.

Ideally, in equation (2), the quantization noise q(t) is uncorrelatedwith the input signal s(t) and closely resembles white noise of powerΔ²/12, where Δ is the quantizer step size (see FIG. 2) as long as theinput signal is limited such that the quantizer operates in theno-overload region. In this case, the two terms that constitute y(t) inequation (1) are uncorrelated, or, equivalently, y_(q)(t) closelyresembles white noise, uncorrelated with the input, and filtered by thehigh pass filter NTF(s). In this case, since NTF(s) is deterministic,the power of the quantization noise measured over a given signalband-width, f_(c), of the ADC output y(t) can be determined usingstandard linear systems analysis as $\begin{matrix}{P_{n} = {\int_{f = 0}^{f = f_{c}}{\frac{\Delta^{2}}{12}{{{NTF}\left( {\mathbb{e}}^{{j2\pi}\; f} \right)}}^{2}\ {{\mathbb{d}f}.}}}} & (4)\end{matrix}$

For a given known input signal power, P_(s), the signal-to-noise ratio(SNR)—a measure of the quality of the analog-to-digital conversionprocess—can then be calculated a-priori according to $\begin{matrix}{{SNR} = {\frac{P_{s}}{P_{n}}.}} & (5)\end{matrix}$

Some properties of the ideal CTΣΔADC, where q(t) resembles white andrandom noise, follow from (4) and (5). For a given fixed f_(c), whichdepends upon the particular application, the SNR depends upon the inputas would be expected from a linear system with q(t) contributingconstant noise power at the output. In other words, any change of signalpower leads to an identical change of SNR in the ADC output. Forexample, suppose that the signal power is doubled, e.g., increases by 6dB, it then follows from (5) that the SNR increases by 6 dB.

Being able to a-priori reliably predict the SNR of the analog-to-digitalconverted signal, as in equations (4) and (5), is extremely important inalmost all applications. Having a-priori knowledge of the SNR deliveredby the ADC to within tight tolerances allows system designers toquantify the performance and behavior of the overall system under avariety of different operating conditions. In practice, in order toproduce the SNR needed for accurate digital processing of the inputsignal s(t), a digital filter is used to filter out frequency componentsabove f_(c) in the ADC output signal. As a result of this filteringprocess, the coarsely quantized output of the CTΣΔADC undergoes asignificant increase in bit-resolution.

As it is obviously desirable to maintain a maximum signal-to-noiseratio, it is commonplace to optimize a particular ADC to compensate forvarious non-changing signal conditions to process a maximum signalswing. Accordingly, if a signal characteristic changes, the performanceof the ADC may be reduced and, alternatively, the signal-to-noise ratioof the ADC may be reduced. For example, a single signal source may, fora variety of reasons, have changing levels of a DC component of a signaldue to circuit operational mode or conditions. Alternatively, a singleADC may be used to sample signals from one of a plurality of signalsources. Each signal source may well have differing DC componentsthereby negatively affecting the performance of the ADC. Due to powerand dye footprint restrictions in wireless transceivers, a single ADCmay be used for multiple applications. Such reuse of the ADC, however,creates the problems previously mentioned. Thus, a need exists for anADC architecture that provides adaptability to input signal DC componentvariations to maintain a maximal signal-to-noise ratio and, moregenerally, to provide for optimal ADC performance.

SUMMARY OF THE INVENTION

The present invention employs a mixture of digital signal processing andanalog circuitry to substantially improve the linear behavior of thebasic low-order CTΣΔADC architectures. In one embodiment of theinvention, a programmable current source that may sink or source currentis coupled across an input resistor to sink or source current from aninput node of the CTΣΔADC. The amount of current that is produced orsunk by the current source and a value of the input resistive valueselectably programmable and may be used to compensate for DC currentcomponents of the signal s(t) and to compensate for signal swing toavoid overloading the operational amplifier based on changes in theinput signal.

The programmable resistor generating a programmable input signalcurrent, i_(s)(t) according to${i_{s}(t)} = {\frac{{s(t)} - V_{B}}{R_{I}}.}$

The operational amplifier of an integrator of the CTΣΔADC keeps avoltage at the negative terminal of the amplifier equal to the constantbias voltage at the positive terminal, V_(B). This offset current alsois programmable. The input signal may be written as the sum of a biasterm or, equivalently, a DC term and an AC term, as follows:s(t)=s _(DC) +s _(AC)(t).

Therefore, the input current signal has a DC component of$\begin{matrix}{i_{DC} = {\frac{S_{DC}}{R_{I}}.}} & (8)\end{matrix}$The parameters are thus selected to compensate for a offset bias currentcomponent of an input signal.

A method of the embodiments of the present invention comprise receivingan analog signal at an input of the CTΔΣADC, producing a scaled inputcurrent based on the received analog signal, wherein the scaled inputcurrent substantially matches the full scale input of the CTΔΣADC, andgenerating a bias current that substantially cancels an offset biascurrent component of the input current. In one embodiment, this stepincludes selecting and coupling a variable bias resistance value betweenthe integrator input and one of a supply voltage and a circuit common.The method further includes integrating the input current to produce anintegrated signal representing a time averaged value of the inputcurrent to substantially remove noise from a frequency band of interest.

The method further includes coupling the integrated signal to aquantizer to produce a digital representation of the scaled analogsignal coupling the digital representation of the scaled analog signalto a digital-to-analog converter (DAC) to produce a feedback currentthat substantially cancels a quantization noise component in the digitalrepresentation of the scaled analog signal. Producing the feedbackcurrent comprises coupling the digital representation of the scaledanalog signal to a programmable digital switch wherein the programmabledigital switch either sinks current from or sources current to theintegrator input.

Other aspects of the present invention will become apparent with furtherreference to the drawings and specification, which follow.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the present invention can be obtained when thefollowing detailed description of the preferred embodiment is consideredwith the following drawings, in which:

FIG. 1 shows an example top-level block diagram of a first-order lowpass CTΣΔADC;

FIG. 2 shows one implementation of the 2-bit quantizer that produces a2-bit feedback to a DAC;

FIG. 3 shows an alternative model of the first-order CTΣΔADC thatincludes an additive noise source q(t);

FIG. 4 is a functional block diagram illustrating a communication systemthat includes a plurality of base stations or access points, a pluralityof wireless communication devices and a network hardware component;

FIG. 5 is a schematic block diagram illustrating a wirelesscommunication device that includes a host device and an associatedradio;

FIG. 6 shows a second example of an ADC in a wireless radio applicationfor monitoring a battery level;

FIG. 7 is a functional block diagram of a radio transmitter formedaccording to one embodiment of the present invention;

FIG. 8 is a functional schematic diagram of a first-order CTΣΔADC formedaccording to one embodiment of the present invention;

FIG. 9 shows an example input signal to the CTΣΔADC;

FIG. 10 shows the simulated power spectral density (PSD) of the CTΣΔADCoutput;

FIG. 11 shows the simulated SNR over a 400 kHz bandwidth;

FIG. 12 is an example of the PSD of the CTΣΔADC output when the ADC isoperating in the overload region;

FIG. 13 shows three different example input ranges that can be achievedfor the CTΣΔADC;

FIG. 14 shows the full scale signal for the CTΣΔADC;

FIG. 15 shows the full scale signal for the CTΣΔADC;

FIG. 16 shows an example implementation of a simple first-order CTΣΔADCin accordance with the present invention;

FIG. 17 shows an example implementation of a simple first-order CTΣΔADCwherein a bias resistor R_(B) is connected to ground.

FIG. 18 shows a second order CTΣΔADC with dual inputs;

FIG. 19 shows a second order CTΣΔADC with dual inputs corresponding toin-phase and quadrature (I/Q) components of a wireless RF receiver; and

FIG. 20 is a flow chart illustrating one method of the presentinvention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 4 is a functional block diagram illustrating a communication system10 that includes a plurality of base stations or access points 12–16, aplurality of wireless communication devices 18–32 and a network hardwarecomponent 34. The wireless communication devices 18–32 may be laptophost computers 18 and 26, personal digital assistant hosts 20 and 30,personal computer hosts 24 and 32 and/or cellular telephone hosts 22 and28. The details of the wireless communication devices will be describedin greater detail with reference to FIG. 5.

The base stations or access points 12–16 are operably coupled to thenetwork hardware component 34 via local area network (LAN) connections36, 38 and 40. The network hardware component 34, which may be a router,switch, bridge, modem, system controller, etc., provides a wide areanetwork (WAN) connection 42 for the communication system 10. Each of thebase stations or access points 12–16 has an associated antenna orantenna array to communicate with the wireless communication devices inits area. Typically, the wireless communication devices 18–32 registerwith the particular base station or access points 12–16 to receiveservices from the communication system 10. For direct connections (i.e.,point-to-point communications), wireless communication devicescommunicate directly via an allocated channel.

Typically, base stations are used for cellular telephone systems andlike-type systems, while access points are used for in-home orin-building wireless networks. Regardless of the particular type ofcommunication system, each wireless communication device includes abuilt-in radio and/or is coupled to a radio. Any one of the wirelesscommunication devices of FIG. 4 may employ the adaptable ADC of thepresent invention as is described in greater detail below.

FIG. 5 is a schematic block diagram illustrating a wirelesscommunication device that includes the host device 18–32 and anassociated radio 60. For cellular telephone hosts, the radio 60 is abuilt-in component. For personal digital assistants hosts, laptop hosts,and/or personal computer hosts, the radio 60 may be built-in or anexternally coupled component.

As illustrated, the host device 18–32 includes a processing module 50,memory 52, a radio interface 54, an input interface 58 and an outputinterface 56. The processing module 50 and memory 52 execute thecorresponding instructions that are typically done by the host device18–32. For example, for a cellular telephone host device, the processingmodule 50 performs the corresponding communication functions inaccordance with a particular cellular telephone standard.

The radio interface 54 allows data to be received from and sent to theradio 60. For data received from the radio 60 (e.g., inbound data), theradio interface 54 provides the data to the processing module 50 forfurther processing and/or routing to the output interface 56. The outputinterface 56 provides connectivity to an output device such as adisplay, monitor, speakers, etc., such that the received data may bedisplayed. The radio interface 54 also provides data from the processingmodule 50 to the radio 60. The processing module 50 may receive theoutbound data from an input device, such as a keyboard, keypad,microphone, etc., via the input interface 58 or generate the dataitself. For data received via the input interface 58, the processingmodule 50 may perform a corresponding host function on the data and/orroute it to the radio 60 via the radio interface 54.

Radio 60 includes a host interface 62, a digital receiver processingmodule 64, an analog-to-digital converter 66, a filtering/gain module68, a down-conversion module 70, a low noise amplifier 72, a receiverfilter module 71, a transmitter/receiver (Tx/RX) switch module 73, alocal oscillation module 74, memory 75, a digital transmitter processingmodule 76, a digital-to-analog converter 78, a filtering/gain module 80,an IF mixing up-conversion module 82, a power amplifier 84, atransmitter filter module 85, and an antenna 86. The antenna 86 isshared by the transmit and receive paths as regulated by the Tx/Rxswitch module 73. The antenna implementation will depend on theparticular standard to which the wireless communication device iscompliant.

The digital receiver processing module 64 and the digital transmitterprocessing module 76, in combination with operational instructionsstored in memory 75, execute digital receiver functions and digitaltransmitter functions, respectively. The digital receiver functionsinclude, but are not limited to, demodulation, constellation demapping,decoding, and/or descrambling. The digital transmitter functionsinclude, but are not limited to, scrambling, encoding, constellationmapping, and/or modulation. The digital receiver and transmitterprocessing modules 64 and 76, respectively, may be implemented using ashared processing device, individual processing devices, or a pluralityof processing devices. Such a processing device may be a microprocessor,micro-controller, digital signal processor, microcomputer, centralprocessing unit, field programmable gate array, programmable logicdevice, state machine, logic circuitry, analog circuitry, digitalcircuitry, and/or any device that manipulates signals (analog and/ordigital) based on operational instructions.

Memory 75 may be a single memory device or a plurality of memorydevices. Such a memory device may be a read-only memory, random accessmemory, volatile memory, non-volatile memory, static memory, dynamicmemory, flash memory, and/or any device that stores digital information.Note that when the digital receiver processing module 64 and/or thedigital transmitter processing module 76 implements one or more of itsfunctions via a state machine, analog circuitry, digital circuitry,and/or logic circuitry, the memory storing the corresponding operationalinstructions is embedded with the circuitry comprising the statemachine, analog circuitry, digital circuitry, and/or logic circuitry.Memory 75 stores, and the digital receiver processing module 64 and/orthe digital transmitter processing module 76 executes, operationalinstructions corresponding to at least some of the functions illustratedherein.

In operation, the radio 60 receives outbound data 94 from the hostdevice 18–32 via the host interface 62. The host interface 62 routes theoutbound data 94 to the digital transmitter processing module 76, whichprocesses the outbound data 94 in accordance with a particular wirelesscommunication standard (e.g., IEEE 802.11a, IEEE 802.11b, Bluetooth,etc.) to produce digital transmission formatted data 96. The digitaltransmission formatted data 96 will be a digital baseband signal or adigital low IF signal, where the low IF signal typically will be in thefrequency range of one hundred kilohertz to a few megahertz.

The digital-to-analog converter 78 converts the digital transmissionformatted data 96 from the digital domain to the analog domain. Thefiltering/gain module 80 filters and/or adjusts the gain of the analogbaseband signal prior to providing it to the up-conversion module 82.The up-conversion module 82 directly converts the analog baseband signalor low IF signal into an RF signal based on a transmitter localoscillation 83 provided by local oscillation module 74. Localoscillation module 74 is, in one embodiment of the invention, amulti-stage mixer as described herein. The power amplifier 84 amplifiesthe RF signal to produce an outbound RF signal 98, which is filtered bythe transmitter filter module 85. The antenna 86 transmits the outboundRF signal 98 to a targeted device, such as a base station, an accesspoint and/or another wireless communication device.

The radio 60 also receives an inbound RF signal 88 via the antenna 86,which was transmitted by a base station, an access point, or anotherwireless communication device. The antenna 86 provides the inbound RFsignal 88 to the receiver filter module 71 via the Tx/Rx switch module73, where the Rx filter module 71 bandpass filters the inbound RF signal88. The Rx filter module 71 provides the filtered RF signal to low noiseamplifier 72, which amplifies the inbound RF signal 88 to produce anamplified inbound RF signal. The low noise amplifier 72 provides theamplified inbound RF signal to the down-conversion module 70, whichdirectly converts the amplified inbound RF signal into an inbound low IFsignal or baseband signal based on a receiver local oscillation 81provided by local oscillation module 74. Local oscillation module 74 is,in one embodiment of the invention, a multi-stage mixer as describedherein. The down-conversion module 70 provides the inbound low IF signalor baseband signal to the filtering/gain module 68. The filtering/gainmodule 68 may be implemented in accordance with the teachings of thepresent invention to filter and/or attenuate the inbound low IF signalor the inbound baseband signal to produce a filtered inbound signal.

The analog-to-digital converter 66 converts the filtered inbound signalfrom the analog domain to the digital domain to produce digitalreception formatted data 90. The digital receiver processing module 64decodes, descrambles, demaps, and/or demodulates the digital receptionformatted data 90 to recapture inbound data 92 in accordance with theparticular wireless communication standard being implemented by radio60. The host interface 62 provides the recaptured inbound data 92 to thehost device 18–32 via the radio interface 54.

As one of average skill in the art will appreciate, the wirelesscommunication device of FIG. 5 may be implemented using one or moreintegrated circuits. For example, the host device may be implemented ona first integrated circuit, while the digital receiver processing module64, the digital transmitter processing module 76 and memory 75 may beimplemented on a second integrated circuit, and the remaining componentsof the radio 60, less the antenna 86, may be implemented on a thirdintegrated circuit. As an alternate example, the radio 60 may beimplemented on a single integrated circuit. As yet another example, theprocessing module 50 of the host device 18–32 and the digital receiverprocessing module 64 and the digital transmitter processing module 76may be a common processing device implemented on a single integratedcircuit. Further, memory 52 and memory 75 may be implemented on a singleintegrated circuit and/or on the same integrated circuit as the commonprocessing modules of processing module 50, the digital receiverprocessing module 64, and the digital transmitter processing module 76.

The wireless communication device of FIG. 5 is one that may beimplemented to include either a direct conversion from RF to basebandand baseband to RF or for a conversion by way of a low intermediatefrequency. In either implementation, however, for the up-conversionmodule 82 and the down-conversion module 70, it is required to provideaccurate frequency conversion. For down-conversion module 70 andup-conversion module 82 to accurately mix a signal, however, it isimportant that local oscillation module 74 provide an accurate localoscillation signal for mixing with the baseband or RF by up-conversionmodule 82 and down-conversion module 70, respectively. Accordingly, thelocal oscillation module 74 includes circuitry for adjusting an outputfrequency of a local oscillation signal provided therefrom. As will beexplained in greater detail below, the local oscillation module 74includes a multi-stage that receives a frequency correction input thatit uses to adjust an output local oscillation signal to produce afrequency corrected local oscillation signal output. While oneembodiment of the present invention includes local oscillation module74, up-conversion module 82 and down-conversion module 70 that areimplemented to perform direct conversion between baseband and RF, it isunderstood that the principles herein may also be applied readily tosystems that implement an intermediate frequency conversion step at alow intermediate frequency.

Within host device 18–32, as shown in FIG. 5, multiple applications foran ADC exist. First, a received RF must be converted to a digital signalby an ADC, such as ADC 66, for subsequent processing by digital receiverprocessing module 64. Additionally, however, ADCs may also be used forproviding signal magnitude and phase information to logic or to aprocessing module, such as a front end processor, for circuitcalibration purposes.

FIG. 6 shows a second example of an ADC in a wireless radio applicationfor monitoring a battery level. The digital baseband processor of FIG. 6leads the battery voltage level via the ADC in regular intervals andgenerates alerts if a battery level goes below critical levels. Othersimilar monitoring applications, such as temperature sensing, may beemployed by these low power ADCs in wireless transceivers.

FIG. 7 is a functional block diagram of a radio transmitter formedaccording to one embodiment of the present invention. FIG. 7 shows butone of a large number of applications for an ADC formed according to anexemplary embodiment of the invention. A radio transmitter 100 includesa digital processor 102 that produces digital data that define a phaseand a frequency of a phase modulated signal. A digital-to-analogconverter (DAC) module 106 is coupled to receive the digital data andproduces a continuous waveform signal to a filter 108. Filter 108produces a filtered analog signal as a reference signal to a phasefrequency detector (PFD) 110. In one embodiment of the invention, thedigital rate (data??) is produced with a high sample rate such that,when converted to analog and filtered, an intermediate frequency (IF)signal is produced. The filtered signal, which may be represented ascos(ω₂₆t+θ_(BB)), is a continuous waveform signal having a frequency of26 MHz. In the described embodiment, the frequency of oscillation is 26MHz, though the output frequency is a function of the digitized IFsignal produced by digital processor 102. Not only is the frequency ofthe filtered IF signal produced by filter 108 determined by digitalprocessor 102, but also the phase, as defined by in-phase and quadraturecomponent values. Accordingly, when radio transmitter 100 is a GSMtransmitter, digital processor 102 further defines a phase of thefiltered IF signal (θ_(BB)) as a part of phase modulating the signalthat is ultimately radiated as a radio frequency transmit signal.

Alternatively, the digital data may be produced at a sample rate suchthat, when converted to analog and filtered, produces one of a basebandor low IF signal. Circuits for up-converting the baseband or low IFsignal are known. Regardless of the frequency of the filtered analogsignal, the PFD 110 of FIG. 7 receives the filtered analog signal andproduces control signals to a charge pump (CP) 112 that, responsive tothe control signals, produces a corresponding error current signal. Aloop filter 114 is coupled to receive the error current signal and toproduce a corresponding error voltage signal to a MUX 115 that, in turn,produces the error voltage signal to a voltage controlled oscillator(VCO) 116. The VCO 116 produces an oscillation, which here also is theRF transmit signal. In the described embodiment, the RF transmit signalproduced by VCO 116 is produced to a power amplifier 118 foramplification and radiation from an antenna.

In the specific embodiment of FIG. 7, radio transmitter 100 is aGSM-based radio transmitter. Accordingly, the output oscillation orcarrier frequency of the RF transmit signal produced by VCO 116 is equalto one of 1800 or 1900 MHz. Power amplifier 118 then receives the 1800or 1900 MHz GSM phase modulated signal for amplification. Within the GSMdomain, however, other frequencies of interest are 850 and 900 MHz.Accordingly, as may be seen, a divide-by-2 module 120 is coupled toreceive the RF transmit signal produced by VCO 116 and produces one of a900 MHz signal or an 850 MHz signal according to whether the RF transmitsignal was a 1900 MHz signal or an 1800 MHz signal. The output ofdivide-by-2 module 120 is then received by power amplifier 122 thatamplifies the signal for propagation from an antenna.

For the purposes of the present example, assume that VCO 116 produces anoutput frequency oscillation of 1800 MHz as the RF transmit signal.Accordingly, divide-by-2 module 120 produces a 900 MHz signal to poweramplifier 122. The 900 MHz signal is further produced to a mixer 124that is further coupled to receive a 926 MHz signal from a FRAC-N phaselocked loop (PLL) frequency synthesizer 126. As is known by one ofaverage skill in the art, mixer 124 multiplies or mixes the two inputsignals, here 900 MHz and 926 MHz, to produce a 26 MHz output signal.The 26 MHz output signal is produced to a feedback filter 128 thatfilters the 26 MHz signal to produce a 26 MHz feedback signal that maybe represented as COS (ω₂₆t+θ_(DCS)/2). The feedback signal is producedto PFD 110 that compares the phase of the feedback signal to thefiltered IF signal (the reference signal) to cause the output phase ofthe RF transmit signal produced by VCO 116 to track the phase of thefiltered IF signal that was produced from the digitized IF signalgenerated by digital processor 102.

In analyzing the feedback signal produced by feedback filter 128, onemay note that the frequency is 26 MHz for the described embodiment.Additionally, the phase modulation index, represented by θ_(DCS/)2,generally illustrates that the phase modulation index has been dividedby 2. This phase modulation index is divided by 2 by the divide-by-2module 120. Divide-by-2 module 120 not only divides the frequency by 2,but also the phase modulation index. Accordingly, as will be describedin greater detail below, digital processor 102 selectively adjusts thephase modulation index according to whether the RF transmit signal isoutput before or after the divide-by-2 module 120. More specifically, ifthe RF transmit signal is amplified and propagated by power amplifier118, then digital processor 102 adjusts the phase modulation index byone-half. If power amplifier 118 is turned off and the RF transmitsignal is divided by 2, and the phase modulation index is divided by 2,by divide-by-2 module 120 prior to amplification and transmission frompower amplifier 122, digital processor 102 does not adjust the phasemodulation index.

Above it was mentioned that FRAC-N PLL frequency synthesizer 126produces a 926 MHz signal to mixer 124. The output of mixer 124,therefore, is a 26 MHz signal. It is understood, of course, that theoutput frequency provided by FRAC-N PLL frequency synthesizer 126 willbe a function of the output frequency provided by the divide-by-2 module120. As is known by one of average skill in the art, a mixer, such asmixer 124, will output a frequency reflecting a difference of the twoinput frequencies. Accordingly, the frequency of FRAC-N PLL frequencysynthesizer 126 is selected so that, when mixed with the output ofdivide-by-2 module 120, a desired frequency feedback signal (here, 26MHz) is produced to feedback filter 128.

An ADC 130, formed according to one embodiment of the present invention,also is coupled to receive the output error voltage signal of loopfilter 114. ADC 130 converts the error voltage signal to a digitalsignal that is produced to a calibration state machine 132 that, basedupon the digital signal produced by ADC 130, generates control signalsto at least one of CP 112, MUX 115 and VCO 116 to adjust operation ofthe translational loop of FIG. 3. Operation of calibration state machine132 is described herein to illustrate one potential application of aninventive ADC such as ADC 130. Generally, low order ADCs having widedynamic range as disclosed herein may be used through a system forconverting analog signals to digital for use in signal process control.

FIG. 8 is a functional schematic diagram of a first order CTΣΔADC formedaccording to one embodiment of the present invention. The circuit ofFIG. 8 shows conceptual mechanisms for input range programmability. Asmay be seen, an input s(t) is produced to a variable resistor R_(i)prior to being produced to a negative terminal of an operationalamplifier. Additionally, a current source that may sink or sourcecurrent is further coupled to the negative input node of the operationalamplifier. The amount of current that is produced or sunk by currentsource I_(B) and the amount of the resistive value of R_(i) areselectably programmable and may be used to compensate for DC currentcomponents of the signal s(t) and to compensate for signal swing toavoid overloading the operational amplifier based on changes in theinput signal.

The arrow over the input resistor R₁ indicates a programmable resistorgenerating a programmable input signal current, i_(s)(t) according to$\begin{matrix}{{i_{s}(t)} = {\frac{{s(t)} - V_{B}}{R_{I}}.}} & (6)\end{matrix}$

The operational amplifier of the integrator keeps the voltage at thenegative terminal equal to the constant bias voltage at the positiveterminal, V_(B). Also included in FIG. 8 is an input offset currentgenerator that generates an offset current I_(B). This offset currentmay also be programmable. The input signal may be written as the sum ofa bias term or, equivalently, a DC term and an AC term, as follows:s(t)=s_(DC) +s _(AC)(t).  (7)

Therefore, from (6) one may determine that the input current signal hasa DC component of $\begin{matrix}{i_{DC} = {\frac{S_{DC}}{R_{I}}.}} & (8)\end{matrix}$

Thus, to find the bias point of the input signal, it is noted that thetotal sum of bias currents going into the input node must be zero, i.e.,I _(B) +i _(DC)=0or, equivalently,s _(DC) =I _(B) R ₁.  (9)

FIG. 9 shows an example input signal to the CTΣΔADC of FIG. 8 where IB=0and R₁ and the DAC are such that the input and feedback currents areequal for equal inputs, i.e.,i _(s)(t)|_(s(t)=1) =i _(fb)(t)|_(y(t)=1).  (10)

The input resistor that satisfies this requirement is referred to asR_(REF).  (11)

Specifically, the signal is a 125 kHz sinusoid of amplitude 1.25V andthus is of the form (7) wheres_(DC)=0; s_(AC)(t)=1.25 sin (2π125e ³).  (12)

FIG. 10 shows the simulated power spectral density (PSD) of the CTτΔADCoutput when driven by the signal defined in (10). Clearly, thefirst-order highpass quantization noise shaping discussed in the aboveis observed.

FIG. 11 shows the simulated SNR over a 400 kHz bandwidth calculated asdefined in (4) and (5). For a large range of input voltages, the SNR(measured in dB) increases linearly with input power measured in dB, asdesired. At some point, around an input voltage of 1.25V, the ADCoverloads, and the SNR decreases rapidly with increasing inputamplitude. Thus, the peak SNR of this ADC is approximately 34.5 dB andis reached with an input amplitude of 1.25V. This signal level isreferred to as the full scale signal.

FIG. 12 is an example of the PSD of the CTΣΔADC output when the ADC isoperating in the overload region. As can be seen, multiple spurioustones arise and the uniform high pass shaping of the quantization noisebreaks down, resulting in poor SNR.

FIG. 13 shows three different example input ranges that can be achievedfor the CTΣΔADC of FIG. 7 without sacrificing SNR by applying thecircuitry specified in the above. Input range 1, i.e., the range[−1.25; 1.25] Volts

-   -   corresponds to the input range in the above example. Input range        2, i.e., the range        [−0.25; 2.25] Volts    -   corresponds to input range 1 with a DC offset of +1.0Volt. Thus,        according to (9), an input offset current generator must be        applied such that $\begin{matrix}        {I_{B} = {- {\frac{1.0}{R_{I}}.}}} & (13)        \end{matrix}$

FIG. 14 shows the full scale signal for the CTΣΔADC of FIG. 7 whereI_(B) satisfies (13). Specifically, a DC offset of +1.0 is observed,corresponding to the desired offset.

Returning to FIG. 13, as can be seen, input signal range example 3 isthe range[−1.125; 0.125] Volts.

Thus, this input range has a DC offset of −0.5 Volts and is only halfthe magnitude of the example input range 1. Thus, according to (9),(10), and (11), the input resistor and bias current generator mustsatisfy $\begin{matrix}{{R_{I} = {\frac{R_{REF}}{2}\mspace{14mu}{and}}}{I_{B} = {{- \frac{1.0}{R_{I}}} = {- {\frac{2.0}{R_{REF}}.}}}}} & (14)\end{matrix}$

FIG. 15 shows the full scale signal for the CTΣΔADC of FIG. 8 where R₁and I_(B) satisfy (14). Specifically, a DC offset of −0.5 and inputrange magnitude of 1.25 is observed, as desired.

FIG. 16 shows an example implementation of a simple first-order CTτΔADCin accordance with the present invention. The bias voltage of thepositive operational amplifier terminal is equal to half the supplyvoltage, i.e., V_(B)=V_(dd)/2. This CTΣΔADC employs a single-bitquantizer and the feedback currents are thus binary. If the ADC outputis “high”, transistor T2 is on and T1 is off and a negative feedbackcurrent flows into the summing node via R_(FB). If the ADC output is“low”, transistor T1 is on, T2 is off, and positive feedback currentflows into the summing node via R_(FB). The resistor R_(B) provides anoffset current to generate a DC offset on the input range. In thisfigure, R_(B) is connected to V_(dd), resulting in positive current flowinto the operational amplifier summing node; hence, according to (9), anegative input range offset is generated. Two switchable inputresistors, R₁ and R₂, provide the ability to switch between twodifferent input range magnitudes.

The first-order CTΣΔADC shown in FIG. 17 is similar to the one shown inFIG. 16, except the bias resistor R_(B) is connected to ground. Thisprovides a negative bias current into the summing node of theoperational amplifier, hence, according to (9), a positive input rangeDC offset results.

The results of the above analysis can readily be extended to higherorder CTΣΔADCs. For example, FIG. 18 shows a second order CTΣΔADC withdual inputs. These inputs correspond to in-phase and quadrature (I/Q)components of the wireless RF receiver shown in FIG. 19. As for the caseof the single-input ADCs discussed above, the input bias currentgenerator I_(B) generates an offset on the input signal ranges. In thiscase, however, the input DC offset on each input branch is given by$\begin{matrix}{S_{DC} = {- \frac{I_{B}R_{I}}{2}}} & (15)\end{matrix}$

-   -   since each input contributes equally in canceling the bias        current.

FIG. 20 is a flow chart illustrating one method of the presentinvention. A first step comprises receiving an analog signal at an inputof the CTΔΣADC (step 400). Additionally, the method includes producing ascaled input current based on the received analog signal, wherein thescaled input current substantially matches the full scale input of theCTΔΣADC (step 420). Producing the scaled input current comprisesselecting and coupling a programmable input resistance value between theCTΔΣADC input and the integrator input. Thereafter, the method accordingto the present embodiment of the invention includes generating a biascurrent that substantially cancels an offset bias current component ofthe input current (step 404). In one embodiment, this step includesselecting and coupling a variable bias resistance value between theintegrator input and one of a supply voltage and a circuit common. Themethod further includes integrating the input current to produce anintegrated signal representing a time averaged value of the inputcurrent to substantially remove noise from a frequency band of interest(step 406).

The method further includes coupling the integrated signal to aquantizer to produce a digital representation of the scaled analogsignal (step 408). Finally, the method according to the describedembodiment of the invention includes coupling the digital representationof the scaled analog signal to a digital-to-analog converter (DAC) toproduce a feedback current that substantially cancels a quantizationnoise component in the digital representation of the scaled analogsignal (step 410). Producing the feedback current comprises coupling thedigital representation of the scaled analog signal to a programmabledigital switch wherein the programmable digital switch either sinkscurrent from or sources current to the integrator input. In oneembodiment, the magnitude of the digital switch current is selected by aprogrammable feedback resistance operably coupled by the digital switchto one of a supply voltage or a circuit common.

As one of average skill in the art will appreciate, the term“substantially” or “approximately”, as may be used herein, provides anindustry-accepted tolerance to its corresponding term. Such anindustry-accepted tolerance ranges from less than one percent to twentypercent and corresponds to, but is not limited to, component values,integrated circuit process variations, temperature variations, rise andfall times, and/or thermal noise. As one of average skill in the artwill further appreciate, the term “operably coupled”, as may be usedherein, includes direct coupling and indirect coupling via anothercomponent, element, circuit, or module where, for indirect coupling, theintervening component, element, circuit, or module does not modify theinformation of a signal but may adjust its current level, voltage level,and/or power level. As one of average skill in the art will alsoappreciate, inferred coupling (i.e., where one element is coupled toanother element by inference) includes direct and indirect couplingbetween two elements in the same manner as “operably coupled”. As one ofaverage skill in the art will further appreciate, the term “comparesfavorably”, as may be used herein, indicates that a comparison betweentwo or more elements, items, signals, etc., provides a desiredrelationship. For example, when the desired relationship is that signal1 has a greater magnitude than signal 2, a favorable comparison may beachieved when the magnitude of signal 1 is greater than that of signal 2or when the magnitude of signal 2 is less than that of signal 1.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof have been shown by wayof example in the drawings and detailed description. It should beunderstood, however, that the drawings and detailed description theretoare not intended to limit the invention to the particular formdisclosed, but, on the contrary, the invention is to cover allmodifications, equivalents and alternatives falling within the spiritand scope of the present invention as defined by the claims. As may beseen, the described embodiments may be modified in many different wayswithout departing from the scope or teachings of the invention.

1. An integrated circuit radio transceiver, comprising: transceivercircuitry for receiving and transmitting RF signals; a multiple inputprogrammable Continuous-Time Delta-Sigma Analog-to-Digital Converter(CTΔΣADC) coupled to receive an analog signal from a selected sourcewithin the transceiver circuitry, the CTΔΣADC for producing digital databased on the received analog signal, wherein the CTΔΣADC includes: aprogrammable input block operably coupled to receive the analog signaland to produce a scaled analog signal; at least one integrator operablycoupled to receive the scaled analog signal to produce at least oneintegrated output; a quantizer operably coupled to receive the at leastone integrated output and for producing a digital output having adigital value coarsely reflecting an amplitude of the analog signal; atleast one programmable digital-to-analog converter (DAC) operablycoupled to receive and to convert the quantizer digital output to ananalog programmable feedback current coupled to an integrator input; aprogrammable bias current block operably coupled to the integrator inputto produce a programmable bias current to substantially cancel a biasvoltage component of the received analog signal; and logic operablycoupled to select an offset bias voltage level to the CTΔΣADC based onat least one of a received analog signal input magnitude and an offsetbias voltage component of the received analog signal.
 2. The integratedcircuit radio transceiver of claim 1 wherein the programmable inputblock comprises a selectable value input resistive element operablycoupled to receive the analog signal and to produce an input current tothe integrator input, wherein the input current is proportional to thereceived analog signal.
 3. The integrated circuit radio transceiver ofclaim 2 wherein the selectable value input resistive element comprisesone of a resistor and a transistor.
 4. The integrated circuit radiotransceiver of claim 1 wherein the programmable bias current blockcomprises a variable bias resistive element operably coupled to theintegrator input and to a first digital switch, wherein the firstdigital switch is selectively controlled to couple the variable biasresistive element to one of a supply voltage and to a circuit common. 5.The integrated circuit radio transceiver of claim 4 wherein the firstdigital switch and the variable bias resistive element are operablycontrolled by the logic.
 6. The integrated circuit radio transceiver ofclaim 5 wherein the logic comprises a digital processor.
 7. Theintegrated circuit radio transceiver of claim 1 wherein the at least oneprogrammable DAC comprises a variable feedback resistive elementoperably coupled to the integrator input and to a second digital switch,wherein the second digital switch is selectively controlled by the logicto couple the variable feedback resistive element to one of a supplyvoltage and to a circuit common.
 8. The integrated circuit radiotransceiver of claim 7 wherein the variable feedback resistive element'svalue is selected to produce a programmable feedback current magnitudethat substantially cancels a quantization noise component present in thequantizer digital output.
 9. A method for producing a programmable inputrange Continuous-Time Delta-Sigma Analog-to-Digital Converter (CTΔΣADC),the method comprising: receiving an analog signal at an input of theCTΔΣADC; producing a scaled input current based on the received analogsignal, wherein the scaled input current substantially matches a fullscale input of the CTΔΣADC; generating a bias current that substantiallycancels an offset bias current component of the scaled input current;integrating the scaled input current to produce an integrated signalrepresenting a time averaged value of the scaled input current tosubstantially remove noise from a frequency band of interest; couplingthe integrated signal to a quantizer to produce a digital representationof a scaled analog signal; and coupling the digital representation ofthe scaled analog signal to a digital-to-analog converter (DAC) toproduce a feedback current that substantially cancels a quantizationnoise component in the digital representation of the scaled analogsignal.
 10. The method of claim 9 wherein producing the scaled inputcurrent comprises selecting and coupling a programmable input resistancevalue between the CTΔΣADC input and an integrator input.
 11. The methodof claim 9 wherein generating the bias current comprises selecting andcoupling a variable bias resistance value between the integrator inputand one of a supply voltage and a circuit common.
 12. The method ofclaim 9 wherein producing the feedback current comprises coupling thedigital representation of the scaled analog signal to a programmabledigital switch wherein the programmable digital switch either sinkscurrent from or sources current to the integrator input.
 13. The methodof claim 12 wherein a magnitude of the digital switch current isselected by a programmable feedback resistance operably coupled by theprogrammable digital switch to one of a supply voltage or a circuitcommon.
 14. A programmable input range Continuous-Time Delta-SigmaAnalog-to-Digital Converter (CTΔΣADC), comprising: a programmable inputblock operably coupled to receive an analog signal and to produce ascaled analog signal; at least one integrator operably coupled toreceive the scaled analog signal to produce at least one integratedoutput; a quantizer operably coupled to receive the at least oneintegrated output and for producing a digital output having a digitalvalue coarsely reflecting an amplitude of the analog signal; at leastone programmable digital-to-analog converter (DAC) operably coupled toreceive and to convert the quantizer digital output to an analogprogrammable feedback current coupled to an integrator input; aprogrammable bias current block operably coupled to the integrator inputto produce a programmable bias current to substantially cancel a biasvoltage component of the received analog signal; and logic operablycoupled to select an offset bias voltage level to the CTΔΣADC based onat least one of a received analog signal input magnitude and an offsetbias voltage component of the received analog signal.
 15. The CTΔΣADC ofclaim 14 wherein the programmable input block comprises a selectablevalue input resistive element operably coupled to receive the analogsignal and to produce an input current to the integrator input, whereinthe input current is proportional to the received analog signal.
 16. TheCTΔΣADC of claim 15 wherein the selectable value input resistive elementcomprises one of a resistor and a transistor.
 17. The CTΔΣADC of claim14 wherein the programmable bias current block comprises a variable biasresistive element operably coupled to the integrator input and to afirst digital switch, wherein the first digital switch is selectivelycontrolled to couple the variable bias resistive element to one of asupply voltage and to a circuit common.
 18. The CTΔΣADC of claim 17wherein the first digital switch and the variable bias resistive elementare operably controlled by the logic.
 19. The CTΔΣADC of claim 18wherein the logic comprises a digital processor.
 20. The CTΔΣADC ofclaim 14 wherein the at least one programmable DAC comprises a variablefeedback resistive element operably coupled to the integrator input andto a second digital switch, wherein the second digital switch isselectively controlled by the logic to couple the variable feedbackresistive element to one of a supply voltage and to a circuit common.21. The CTΔΣADC of claim 20 wherein the variable feedback resistiveelement's value is selected to produce a programmable feedback currentmagnitude that substantially cancels a quantization noise componentpresent in the quantizer digital output.